Rapid diffusion of wireless devices typified by mobile communications has increased the demand for small and light acoustic wave filters and multiplexers including such acoustic wave filters. Piezoelectric thin film resonators such as surface Acoustic Wave (SAW) resonators, Film Bulk Acoustic Resonators (FBARs), and Solidly Mounted Resonators (SMRs) are widely used as the acoustic wave resonator.
It has been known to couple a capacitor to a duplexer or a filter and couple a band suppression filter such as a notch filter to a duplexer or a filter as disclosed in, for example, Japanese Patent Application Publication Nos. 10-56304 (Patent Document 1), 9-284093 (Patent Document 2), 2012-501564 (Patent Document 3), 2015-154434 (Patent Document 4), 2010-109894 (Patent Document 5), 2005-109573 (Patent Document 6), and 2005-136588 (Patent Document 7), and International Publication No. 2012/176508 (Patent Document 8). It has been known that the piezoelectric thin film resonator easily generates second harmonics as disclosed in Japanese Patent Application Publication No. 2008-85989 (Patent Document 9).
When a capacitor is coupled to a filter including a piezoelectric thin film resonator, use of a chip capacitor increases the filter size. In addition, the chip capacitor has a large tolerance. Thus the variation in capacitance is large. When a capacitor is located on a substrate including a piezoelectric thin film resonator formed thereon, unnecessary spurious such as second harmonics occurs.